Such a method is known from EP-A 322 006 which corresponds to U.S. Pat. No. 4,918,387 and from the magazine "Magnetic Resonance in Medicine" 8, 355-361 (1988). In dependence upon the phase position of the second frequency-selective high-frequency pulse, by means of the known method either lactic acid (lactate) or fat (lipids) can be demonstrated in an examination region containing water besides fat and lactic acid.
The 180.degree. high-frequency pulse is then a so-called hard pulse, whose frequency spectrum includes the Lamor frequency of water. In the ideal case--if this pulse triggers the nuclear magnetization each time everywhere through 180.degree.--the nuclear magnetization of protons bound to water in the examination region is not excited thereby. In practice and more particularly in examinations at the living body, however, small deviations from this angle are obtained, which on account of the comparatively high water concentration in the human body impede the demonstration of the lactate or lipid components or even make this demonstration completely impossible.
It is known for suppressing the disturbing water components to invert in every second sequence the phase position of 180.degree. pulse--related to the first frequency-selective pulse. Thus, the sign of the component contained in the nuclear magnetic resonance signal and leading back to the water is inverted from sequence to sequence so that these components compensate each other after a summation of the nuclear magnetic resonance signals. However, this method is sensitive to movements in the examination region, such as may occur during the spectroscopic examination at the human body.